1. Field of the Invention
The present invention relates to a semiconductor device in which thin film transistors (hereinafter referred to as TFTs) are formed on a substrate having an insulating surface. In particular, the present invention relates to a liquid crystal display device having an active matrix display portion, an EL display device, and other active matrix display devices.
2. Description of the Related Art
The development of electronic equipment having mounted semiconductor devices therein, in particular semiconductor display devices, has been remarkable in recent years. There are many examples of such applications, including portable equipment like game machines, notebook personal computers, and portable telephones, liquid crystal televisions, liquid crystal displays (liquid crystal display devices), and OLED displays (EL display devices). Semiconductor devices and semiconductor display devices can be made lighter and thinner compared to conventional CRTs, and have a characteristic of low electric power consumption.
Active matrix semiconductor display devices in which TFTs are disposed in a matrix shape in a pixel region, and passive matrix display devices in which stripe shape electrodes are formed crossing each other in a pixel region are known as semiconductor display devices.
With active matrix semiconductor devices, logic circuits other than driver circuits for driving pixels (for example, gate signal line driver circuits and source signal line driver circuits) are formed on a silicon substrate or the like that is separate from a pixel substrate, and mounted externally to the pixel substrate. Note that, although the logic circuits are often mounted externally when using amorphous silicon (non-crystalline silicon films) to form TFT active layers on the pixel substrate, it is also possible to integrally form the logic circuits on the pixel substrate if the active layers are formed using polysilicon (crystalline silicon films).
Note that, within this specification, the term pixel substrate indicates a substrate on which a pixel region is formed. More precisely, the term pixel substrate indicates a substrate on which TFTs (pixel TFTs) disposed in each pixel are formed.
The structure of an active matrix semiconductor display device in which amorphous silicon is used in active layers of TFTs (hereinafter also referred to as amorphous TFTs) is shown in FIG. 4. A gate signal line driver circuit (also referred to as a scanning line driver circuit) 101 and a source signal line driver circuit (also referred to as a signal line driver circuit) 102 are mounted to an FPC 103 by a method such as TAB (tape automated bonding), and externally attached to a substrate (pixel substrate) 104 on which a pixel region 100 is formed. Control signals for the gate signal line driver circuit 101 and the source signal line driver circuit 102 are input from an external portion through the FPC 103.
The structure of an active matrix semiconductor display device in which polysilicon is used in forming active layers of TFTs (hereinafter also referred to as polysilicon TFTs) is shown in FIG. 2. A gate signal line driver circuit 203 and a source signal line driver circuit 202 are formed integrally on a pixel substrate 200. Control signals and image data are input to the gate signal line driver circuit and the source signal line driver circuit from outside through an FPC 204.
An example of an active matrix semiconductor display device having conventional polysilicon TFTs (specifically, a liquid crystal display device) is shown in FIG. 3A. A gate signal line driver circuit 302 and a source signal line driver circuit 303 are formed integrally on a substrate 300 on which a pixel portion 304 is formed. The substrate 300 is connected to a system bus 310 through an interface circuit 305. The substrate 300 and the interface circuit 305 are electrically connected by an FPC 321. In addition, an image processing circuit 306, a VRAM 307, a CPU 311, a memory 308, an interface circuit 309 for communicating to external devices, and the like are also connected to the system bus.
Note that the term VRAM indicates a memory for temporarily storing image data. Further, the term interface circuit denotes a circuit such as a signal format converter or amplifier, through which communication with external devices is performed.
FIG. 3B shows a cross sectional diagram cut along a line segment AB of FIG. 3A Further, although the pixel region and the gate signal line driver circuit contain a plurality of TFTs, an n-channel TFT forming the pixel region, and an n-channel TFT and a p-channel TFT forming the gate signal line driver circuit are shown here representatively. Furthermore, reflective and transmittive active matrix liquid crystal display devices exist, and a cross sectional diagram of a reflective active matrix liquid crystal display device is shown here.
Liquid crystals 314 are filled into a space surrounded by a glass substrate 312, an opposing substrate 301, and a sealing agent 315. An interlayer insulating film (leveling film) 318 is formed on TFTs formed on the glass substrate, and a reflecting electrode 317 which is electrically connected to a drain electrode of a pixel TFT is formed on the interlayer insulating film 318. An orientation film 319 is formed on the reflective electrode 317. A leveling film 322 is formed first on the opposing substrate, an opposing electrode 313 made from a transparent electrode (ITO) is formed on the leveling film 322, and an orientation film 320 is formed on the opposing electrode 313.
Sunlight, room light, or other external light is reflected by the reflecting electrode 317 as shown by an arrow in FIG. 3B, thus performing screen display.
Logic circuits other than the gate signal line driver circuit and the source signal line driver circuit are formed on a different substrate from the pixel substrate, and then mounted to the aforementioned semiconductor devices, semiconductor display devices, and in addition, electronic equipment in which the semiconductor display devices are installed.
The miniaturization of electronic equipment is a very important issue accompanying the spread of portable electronics, and miniaturization is difficult with the aforementioned structure in which is it necessary to mount a plurality of IC chips, differing from the pixel region. One reason that can be given is that a margin used for mounting is large, even if a logic circuit within an IC chip can be made small.
On the other hand, if an attempt is made to reduce the mounting margin in order to achieve miniaturization, then a high precision mounting technique becomes necessary, and problems in cost and the reliability of the mounted components develop.